I think I have this issue under control, but just wanted to doublecheck I'm taking the right course of action. My monovalve has entirely quit at this point, in that I get 12v to it but still have constant heat. I tried supplying 12v to the pins myself and get no sound. I have a spare used monovalve on my shelf, I think also from am 87 300D, and when I put 12v to that, I get a substantial click. Can I assume that the coil in my monovalve is kaput? So the whole unit needs to be replaced? Or is the lack of any noise possibly due to the valve being stuck ... in which case I could rebuild it I suppose (I noticed via a search that a lot of people rebuilt them).
In either case, is there any reason not to use the used one I have? It seems to work electronically and I thought I could just swap it in and see if it works.

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1987 300D, arctic white/palomino--314,000 miles
1978 240D 4-speed, Euro Delivery, light ivory/bamboo--370,000 miles
2005 Jeep Liberty CRD Limited, light khaki/slate--140,000 miles
2018 Chevy Cruze diesel, 6-speed manual, satin steel metallic/kalahari--19,000 miles
1982 Peugeot 505 diesel, 4-speed manual, blue/blue, 130,000 miles
1995 S320, black/parchment--34,000 miles (Dad's car)